Heterosis, Combining Ability and Gene Action for Yield in Bottle Gourd

The experiment was conducted at the experimental field of Olericulture Division, Horticulture Research Centre (HRC), Bangladesh Agricultural Research Institute (BARI), Gazipur, Bangladesh during the winter season of 2018-2019 to study the genetic architecture of yield in a seven parent half diallel cross of bottle gourd. The values of mean square for GCA (general combining ability) and SCA (specific combining ability) were highly significant which suggested the presence of both additive and non-additive genetic variance in the population. But the higher magnitude of GCA compared to SCA indicated predominance of additive genetic variance. In most of the cases, the cross between poor and poor parents showed positive SCA effect for fruit yield, which indicated the higher yield. The estimates of significant positive better parent heterosis ranged from 6.27 to 49.72 percent. Analysis of genetic components of variation suggested that additive components were more important in the inheritance of fruit yield. This character was observed being controlled by two to three pairs of genes or groups of genes. Narrow sense heritability was 23 percent indicating probability of selection in generations. The graphical analysis also indicated wide genetic diversity among the parents.

Taiwan, Thailand, South Africa and Sudan [2]. Bottle gourd fruits are used as cooked vegetables. Its leaves and tender stems are used as delicious and nutritious vegetables. It is reported as an easily digestible vegetable which keeps the body cool and prevents constipation [3]. Each 100 g bottle gourd contains protein 1.1 g, carbohydrate 15.1 g, fat 0.1 g, minerals 0.6 g, and several vitamins [4].
Bottle gourd is a popular winter vegetable in Bangladesh. The climatic condition of winter in Bangladesh favours better growth and yield of bottle gourd but hot and humid summer and summer rainy season gives poor yield. The average day temperature of 20˚C -27˚C with lower night temperature of 18˚C -23˚C is optimum for growth and fruiting. Anthesis of flower in different cultivars is influenced by both temperature and rainfall. Bottle gourd is widely cultivated throughout the country. Its cultivation and uses are wide in winter season but nowadays it is cultivating during summer and rainy season. At present, the acreage and annual production of bottle gourd are 7217 ha and 85,267 tons respectively in Bangladesh with an average yield of 11.81 tons per hectare [5], which is very low compared to other countries. Hybrid varieties may play a vital role in satisfying the interest of producers and consumers. The identification and utilization of the most heterotic and useful crosses are very important in hybrid approach to make the commercial cultivation of hybrid beneficial [6]. So a wellplanned and dynamic bottle gourd breeding research programme is needed to meet the required demand of bottle gourd production.
The understanding of the nature and magnitude of gene action is an important factor in developing an effective breeding programme. The diallel analysis provides an efficient means of rapidly obtaining an overall picture of the genetic control of a character in a set of parents in the early generations. The analysis of combining ability helps in selecting suitable genotypes as parents for hybridization and crosses for characterizing the nature and magnitude of gene action involved in quantitative traits [7]. The use of contrasted lines in breeding programmes could contribute to create high yield varieties [8]. On the other hand, plant selection for high yield can be effective only if the variables under selection have high heritability values [9]. In Bangladesh context, the information on this aspect of bottle gourd is not sufficient. This study would be very important in developing hybrid varieties for Bangladesh conditions. Therefore, the present investigation was undertaken to investigate the genetic architecture of yield in bottle gourd.

Materials and Methods
The experiment was conducted at the Olericulture Division of Horticulture Re-

Statistical and Biometrical Analysis
In the analysis of variance (ANOVA) the F test was used at the 5% and 1% levels of probability. The trait means were compared by the Tukey test at the 5% level of probability. The ANOVA for the experiment (RCBD) was estimated according to [11] procedure. The combining ability analysis for studied traits was carried out using method 2 of model 1 of [12], where parents and F 1 's were included under the experiment excluding reciprocals.
Griffing's Method 2 and model 1 is as follows: where u = the population mean; g i = the general combining ability effect of the ith parent; g j = the general combining ability effect of the jth parent; s ij = the specific combining ability effect of the cross between ith and jth parents such that s ij = s ji ; e ijk = the environmental effect associated with ijkth observation.
Thus the experimental mccaterial comprises of ( )

Estimates of GCA and SCA Effects
The GCA and SCA effects were estimated according to [16] by the following formula:

Estimation of Heterosis
For estimation of heterosis in each character the mean values of the 21 F 1 's have been compared with better parent (BP) for heterobeltiosis. Percent heterosis was calculated as: The significance test for heterosis was done by using standard error of the value of better parent as:

Computation of Variance Components and Allied Parameters in F1 Diallel
Using the values of different statistics computed earlier the variance components and allied parameters as follows: Variance components and allied parameters Formulae

Combining Ability
A perusal of results of combining ability analysis indicated considerable nonadditive gene action in the inheritance of majority of the attributes studied. Hence, breeding methods involving selection, intermating of selects and reselection in A. Quamruzzaman et al.
segregating generations followed by pedigree method of breeding may help to improve the characters [7]. The values of mean sum of square for both GCA (general combining ability) and SCA (specific combining ability) were highly significant for yield in bottle gourd, which suggests the presence of both additive and non-additive genetic variance in the population (Table 1)

GCA Effect
In the present study, parent P6 showed the greatest relative GCA effect (1.75**) followed by P1 (1.07*), P2 and P7 (1.06*) for yield. Other parents showed either insignificant negative or significant negative GCA values. So, parents P6, P1, P2 and P7 were the best general combiners in crosses for the improvement of this trait. [17] [18] [19] reported that significant positive GCA effects obtained in bottle gourd. [19] also reported KBG-16 was the best general combiner for total yield per vine. While the parents P4 (−2.74**), P3 and P5 (−1.12*) were the poor combiner for yield in this study.

SCA Effect
Out of 21 cross combinations 14 crosses showed positive SCA effect for yield, among them 12 crosses exhibited significant positive SCA effect ( Table 2). The highest positive significant SCA effects were shown by the hybrid P3 × P4 (6.90**) followed by P3 × P5 (5.12**), P4 × P5 (4.22**), P4 × P7 (3.57**), P2 × P6 (3.40**) and P2 × P7 (3.52**). Thus P3 × P4 was the best combination (poor × poor combiner) similar to other two hybrids (P3 × P5, P4 × P5) for yield in bottle gourd. The other moderately higher values of positive SCA effect may be considered as good specific combiner viz., P4 × P7 (poor × good combiner) and P2 × P6, P2 × P7 (good × good combiner) for fruit yield. [19] also found 3 hybrids viz., GH-10 × G-2, GH-9 × PSPL and GH-13 × G-2 exhibited significant  specific combining ability effects for fruit yield. [17] reported best SCA effect was obtained by 7 crosses and [18] reported best combiner was IC-92362 × Pusa Naveen in bottle gourd, which was in agreement with the present findings. [19] reported that the parents having poor GCA for certain traits when crossed with parents having high GCA for the same traits usually generated high positive SCA effect. Similar trends were also observed in the present study like P4 × P7. The parents like P4 having poor GCA for yield, when crossed with P7 having good GCA, for this character, gave higher positive SCA effects (P4 × P7). So a relationship seems to exist between general and specific combining ability effects, it would safely be assumed that good or poor GCA may provide good SCA combination.

Heterosis
Significant difference between genotype and replication was observed in ANOVA for fruit yield (  [22] reported the best performing hybrids for yield were S36-1 × NC59812-1 and S39-1 × S1-3, with 76.4% and 58.1% heterosis over better parents, respectively. For fruit yield the hybrids ABGS11-23 × DBG-5, ABG-1 × Arka Bahar and DBG-6 × DBG 5 recorded the highest estimates of heterosis over standard check [6]. The yield per plant exhibited appreciably high amount of heterosis over the better parent, top parent and commercial control [24]. These were in agreement with the present findings (Table 3).

Genetic Components of Variation
The components of variation along with the derived genetic ratios for fruit yield (Table 4) showed that the D and H components, which measure additive and dominance variation, respectively were significant. This indicated the importance of both additive and dominance components for the inheritance of all the genotypes in bottle gourd. However, the magnitude of dominance was higher than the additive component. These results agree with that reported by [25]. The H 2 representing dominance deviation due to relative frequency of positive and negative genes was significant. The proportion of positive effects as indicated by F value was non-significant for fruit yield, suggesting greater frequency of dominant alleles governing this character. The net dominance effect, obtained by

Graphical Analysis
Graphical analysis of parent-offspring covariances (Wr) on array variances (Vr) is shown in Figure 1. It was observed from the Wr/Vr graph that the slope of the regression line for fruit yield was significantly below 1.0 (0.36 ± 0.11), suggesting A. Quamruzzaman et al. ings were reported by [31] [32] in bean and [33] in eggplant.

Conclusion
In the present study, the parents, P6, P1, P2 and P7 were found to be good general combiners for yield and may be used in a breeding programme for developing high yielding varieties. The crosses, P3 × P4, P3 × P5, P4 × P5, P4 × P7, P2 × P6 and P2 × P7 were found promising for high yielding. The both additive and dominance components of variation were important for the yield. The genetic components of variation analysis suggested that the excess of dominant alleles and a minority of recessive alleles i.e., presence of asymmetrical distribution for dominant alleles in the parents. Heritability in narrow sense indicated that this character was highly heritable. The yield was observed being controlled by two to three pairs of genes or groups of genes. The graphical analysis indicated wide genetic diversity among the parents.